1. Field of the invention
The present invention relates to an object displacement sensor adapted for high speed counting of passing nonmagnetic objects, regardless of their conductivity.
2. Description of the Prior Art
Sensors described in the prior art, designed to count metallic objects passing by at high speed, use either the magnetic properties of the material composing the objects or their electrical conductivity. Effectively, a mass of ferro-magnetic material passing close by a conductive wire coil with a metal magnet core causes a variation in the magnetic induction in the core and thus a current impulse along the coil. This impulse is then amplified, filtered and put in a form in which it can be counted by logical counting circuits. The electrical conductivity is, for its part, used to exploit the variation in the induction coil which provides the Foucault current inducted in the conductive metal of which the objects are made. The sensitivity of this system is directly associated with the conductivity of the metal.
Sensors of the type just described have many disadvantages which limit their use. In effect, their use requires the presence of a ferro-magnetic element in the counted objects or a weak electrical resistance. Furthermore, in the presence of radio-electric noise more particularly high frequency noise, the conductive wire coil of the sensor generates a current with a noise level frequency high enough to impede precise counting of objects. This radio-electric noise effect is particularly important in structures such as aircraft turbine reactors which it is necessary to measure the rotor speed with precision.